Nested Loop Antennas
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Reprinted with permission from March 2014 QST ARRL, the national association for Amateur Radio ® www.arrl.org G. Scott Davis, N3FJP This multi-band nested loop antenna array replaces my tribander Yagi, which is only up 20 feet. Inspired by suggestions from Bill Wisel, K3KEI, I first tried a full wave 20 meter band square loop antenna. On the air comparisons with my low Yagi confirmed instantly that this design was a hands-down winner for working both local and distant stations. I replaced that mono-band loop with a nested loop array for the 20, 17, 15, 12, and 10 meter bands. The antenna blends into the surroundings, so I needed the morning sun shining directly on it to snap the lead photo. This became a nice father-son project with my son Brad, KB3MNE. Here’s how we built the antenna. Construction We constructed the square loops shown in Figure 1 according to the dimensions in Table 1. The loops hang from a tree limb in the vertical plane. Because I feed them from the bottom corners, the loops radiate horizontal polarization. Calculate the perimeter size, P, of each loop by dividing the frequency in MHz into 1005 feet. Table 1 shows the loop dimensions. Start with the 20 meter loop, the largest loop. Cut a 70.90 foot segment of wire for the loop perimeter length, P. Divide the perimeter length by 4 to de- termine the length, S, of each side. The PVC pipe horizontal cross support spans between opposite loop corners, and is 1.41 times the largest loop side length, S. We used brass screws to hold the PVC together. Lay your antenna wire out on the structure, temporarily taping it to find where the wire should pass through the PVC pipe. Drill Nested Loop Antennas This low-cost five band loop array blends into the background. Table 1 Nested Loop Antenna Dimensions; 75 W Matching Section Cable has 0.66 Velocity Factor Band Frequency, MHz Perimeter, P, ft Side, S, ft Cross Support, ft Distance From Center, D, ft Length, L, of 75 W cable 20 m 14.175 70.90 17.72 24.81 12.41 11.45 17 m 18.118 55.47 13.87 19.41 9.71 8.96 15 m 21.225 47.35 11.84 16.57 9.29 7.65 12 m 24.940 40.30 10.07 14.10 7.05 6.51 10 m 28.500 35.26 8.82 12.34 6.17 5.70 holes through the pipe for the loop wire. After you run the wire through the holes, wrap a bit of electrical tape on each side of the wire next to the pipe to keep the wire from sliding and to give the pipe additional support. Next, cut and thread the wires for the rest of the bands. The horizontal PVC pipe sagged and bowed significantly even though the an- tenna wires help support the horizontal PVC pipe pieces. We remedied the sagging by taping lengths of 1 × 2 inch pressure treated wood to the horizontal arms. At the bottom, rather than sliding the wires through the PVC pipe, we put brass wood screws into the PVC to hang the dipole connectors seen in Figure 2. Matching and Feeding Each loop antenna feed point impedance is about 100 W, so we used individual quar- ter wave 75 W coax matching sections to convert the impedance to 50 W. Carefully cut 75 W coax to the lengths L specified in Table 1. Those lengths, taking into account velocity factor v = 0.66, are L = 246v/F MHz feet. Connect one end of the 75 W coax to the loop using a dipole center connector, as seen in the lead photo. The other end connects to the 50 W coax feed line using This stealthy nested loop is almost invisible among the trees.
Transcript of Nested Loop Antennas
Reprinted with permission from March 2014 QST ARRL, the national association for Amateur Radio® www.arrl.org
G. Scott Davis, N3FJPThis multi-band nested loop antenna array replaces my tribander Yagi, which is only up 20 feet. Inspired by suggestions from Bill Wisel, K3KEI, I first tried a full wave 20 meter band square loop antenna. On the air comparisons with my low Yagi confirmed instantly that this design was a hands-down winner for working both local and distant stations.
I replaced that mono-band loop with a nested loop array for the 20, 17, 15, 12, and 10 meter bands. The antenna blends into the surroundings, so I needed the morning sun shining directly on it to snap the lead photo.
This became a nice father-son project with my son Brad, KB3MNE. Here’s how we built the antenna.
ConstructionWe constructed the square loops shown in Figure 1 according to the dimensions in Table 1. The loops hang from a tree limb in the vertical plane. Because I feed them from the bottom corners, the loops radiate horizontal polarization.
Calculate the perimeter size, P, of each loop by dividing the frequency in MHz into 1005 feet. Table 1 shows the loop dimensions. Start with the 20 meter loop, the largest loop. Cut a 70.90 foot segment of wire for the loop perimeter length, P. Divide the perimeter length by 4 to de-termine the length, S, of each side. The PVC pipe horizontal cross support spans between opposite loop corners, and is 1.41 times the largest loop side length, S. We used brass screws to hold the PVC together. Lay your antenna wire out on the structure, temporarily taping it to find where the wire should pass through the PVC pipe. Drill
Nested Loop AntennasThis low-cost five band loop array blends into the background.
Table 1 Nested Loop Antenna Dimensions; 75 W Matching Section Cable has 0.66 Velocity Factor
Band Frequency, MHz Perimeter, P, ft Side, S, ft Cross Support, ft Distance From Center, D, ft Length, L, of 75 W cable
20 m 14.175 70.90 17.72 24.81 12.41 11.4517 m 18.118 55.47 13.87 19.41 9.71 8.9615 m 21.225 47.35 11.84 16.57 9.29 7.6512 m 24.940 40.30 10.07 14.10 7.05 6.5110 m 28.500 35.26 8.82 12.34 6.17 5.70
holes through the pipe for the loop wire. After you run the wire through the holes, wrap a bit of electrical tape on each side of the wire next to the pipe to keep the wire from sliding and to give the pipe additional support. Next, cut and thread the wires for the rest of the bands.
The horizontal PVC pipe sagged and bowed significantly even though the an-tenna wires help support the horizontal PVC pipe pieces. We remedied the sagging by taping lengths of 1 × 2 inch pressure treated wood to the horizontal arms. At the bottom, rather than sliding the wires through the PVC pipe, we put brass wood
screws into the PVC to hang the dipole connectors seen in Figure 2.
Matching and FeedingEach loop antenna feed point impedance is about 100 W, so we used individual quar-ter wave 75 W coax matching sections to convert the impedance to 50 W. Carefully cut 75 W coax to the lengths L specified in Table 1. Those lengths, taking into account velocity factor v = 0.66, are L = 246v/FMHz feet. Connect one end of the 75 W coax to the loop using a dipole center connector, as seen in the lead photo. The other end connects to the 50 W coax feed line using
This stealthy nested loop is almost invisible among the trees.
QST® – Devoted entirely to Amateur Radio www.arrl.org Reprinted with permission from March 2014 QST
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Photos by the author.
G. Scott Davis, N3FJP, holds an Amateur Extra class license. He was first licensed in 1976 as WB3CZF. Scott shares his interest in Amateur Radio and programming with his wife Kimberly, KA3SEQ, and sons Christopher, KB3KCN, and Bradley, KB3MNE. You can reach Scott at [email protected].
1Available from your ARRL dealer, or from the ARRL Store, ARRL order no. 6621. Telephone toll-free in the US 888-277-5289, or 860-594-0355, fax 860-594-0303; www.arrl.org/shop/; [email protected].
a double barrel female SO-239 connec-tor. I select the appropriate antenna by an antenna switch controller in my ham shack.
ConclusionMy nested loop array works well on all five bands. There is little interaction between
Dacron Rope to Tree Limb
1/2"PVCPipe
Ground Level
Five 50 Ω CoaxLines to Five-wayAntenna Selector
Five 75 Ω CoaxMatching Sections,
Lengths L
Five Loops:Side Lengths S,Perimeters (P)
Cross Piece,Length C
(24.81' for 20 m)
Dipole Connectors
QS1403-Davis01
5.7'
6.51'
7.65'
8.96'11.45'
12.41'
6.71'7.05'
9.29'9.71'
17.72' (70.90')
13.87' (55.47')
11.84' (47.35')
10.07' (40.30')
8.82' (35.26')
Figure 1 — The five-band loop antenna and construction overview.
Figure 2 —The five SO239 dipole connectors attach the loops to 75 W coax quarter wave matching sections. Brad, KB3MNE, points to the 17 meter connector.
Antenna wire — #12 AWG black insulated stranded copper wire, 250 feet (homedepot.com).
Coax cable — 75 W, 41 feet (RG11A/U or RG59B/U www.thewireman.com).
CQ dipole center hang, SO239 connector, 5 units (Part #801 www.thewireman.com).
Double barrel female SO-239 connectors, 5 units
PL259 coax connectors, 10 units.50 feet of 1⁄2" PVC tubing (four 12.5" sections).PVC 4 way connector, 1⁄2".PVC 1⁄2"″end caps, 4 units.Wood stiffeners, 1" × 2" × 8" (optional) pressure
treated wood.Dacron UV resistant 3⁄16" rope (Part #816 www.
thewireman.com).Brass wood screws, #6 x 11⁄2", and PVC cement.
the antenna elements, and the signal re-ports are very encouraging. You can build a single loop for your favorite band, or nest multiple loops. You can even add 6 and 2 meters if you like. [Be sure to see the compliance tables in RF Exposure and You by Ed Hare, W1RFI. — Ed.]1
